This invention relates generally to a pole-shoe magnet structure for a magnetomotive device and particularly to such structure for use in breakerless ignition systems for internal combustion engines.
Generally, breakerless ignition systems for internal combustion engines employ a permanent magnet rotating in synchronism with the crankshaft of the engine. In those ignition systems known as capacitor discharge (C/D) systems, the permanent magnet induces a voltage in a charging winding connected to a capacitor, and also induces a voltage in another coil of the system, generally known as a trigger coil. A solid state switching component such as a transistor or SCR, connected between the capacitor and an ignition coil, conducts periodically in response to the trigger signal whereby the capacitor charge is discharged through the primary of the ignition coil and an ignition pulse is thereby induced in the secondary winding. The voltage induced in the secondary winding which is connected to a spark plug causes the spark plug to fire. For different performance characteristics and/or economy of construction, the triggering signal may be derived from the primary winding or other available coil used in the ignition system.
In typical operation, an SCR is biased well below forward breakdown, voltage and triggering is accomplished by applying to the gate electrode current of predetermined amplitude and polarity. When used in an ignition system, a trigger coil is provided whereby an electrical pulse is generated in response to rotation of the engine shaft. The characteristics of the pulses generated by such trigger coils are important to consider in avoiding the problem of ignition system triggering in the event of reverse engine rotation. In many small engines having breakerless ignition, it has been the practice to provide special safety circuits to prevent ignition firing upon reverse engine rotation. Such circuitry contributes significantly to the cost of the ignition system.
Accordingly, it is a principal object of the present invention to provide a pole-shoe magnet group for a magnetomotive ignition system which overcomes the deficiencies of the prior art.
It is another object of the present invention to provide a pole-shoe magnet group for a magnetomotive ignition system which prevents ignition in the event of reverse rotation of the engine without the need for additional electronic components.
It is another oject of the present invention to provide a pole-shoe magnet group which generates a narrow pulse for accurate ignition timing in breakerless ignition systems.
It is another object of the present invention to provide a pole-shoe magnet group which generates rapid rise trigger pulses of essentially one polarity.